The idea presented for (REF ) is simple and unconditional
stability is a strong conclusion. It is not surprising that it has been
elaborated in various ways. While the authors do not know the first place
appeared (one author saw the idea used in a presentation in 1980),
elaborations include [1]}, [2]}, [3]} and recent work
on convex splitting methods of Gao, Jin and Liu [4]} and Liu, Wang and
Zhou [5]}. Finding \(\mathcal {O}(k^{2})\)  extensions of
(REF ) with the same unconditional stability result is
nontrivial. The only step we are aware of (aside from defect/deferred
corrections wrapped around the first order approximation) is Trenchea
[6]}.
